There are four things a gasoline engine requires to generate power and almost any engine problem will come down to something relating to one of these four factors. Let’s focus on each one of these topics individually, and also look into friction:
Dirty Air Filter
Luckily an easy fix, air filters can get clogged up with debris over the miles, so it’s important to either clean them or replace them when this happens. On most cars it’s a simple check, and often doesn’t even require tools. A clogged air filter makes the engine work harder to pull in air, and can restrict the maximum amount of air let in. Less air means less power.
What comes in must go out. Restrictions on either end of the block will mean reduced airflow and reduced performance. The engine will have to work harder to push out exhaust gases, limiting power. Catalytic converters can clog up on engines with an improper air/fuel mixture or as a result of fuel additives reacting within. Mufflers can also fail internally, whether from rust or other factors, and alterations of the internal piping can result in an airflow restriction.
This is especially true for engines that aren’t self-adjusting. Over time the valve train components wear (this can be minimized by using the proper engine oil and changing regularly, but regardless wear will occur). As the components which are responsible for the opening and closing of the valves wear, it’s possible for the valve timing and valve lift to be reduced. Less valve lift and less duration means less airflow, especially on the top end, so it’s important to adjust the valves to compensate for the wear.
Anything that restricts fuel from entering the cylinder will cause a loss of power if the engine requires more fuel than is injected.
Clogged fuel injectors create a cascade of problems. Deposits can build up on the injectors over time from heat soak or poor fuel. Small restrictions can cause the O2 sensors to read a lean mixture, and so more fuel will be added to compensate. This can result in a rich mixture for the cylinders without injector problems (which has its own consequences), or even misfiring if the injector isn’t capable of injecting enough fuel. Ultimately, you want proper control of fuel injection for maximum performance.
Overtime the fuel pump can wear out, but it might not necessarily fail catastrophically. While it may still be able to supply fuel at lower pressures, it may begin to struggle to provide fuel at higher pressures or for longer durations. If your vehicle loses power under heavy acceleration, or traveling uphill, or sputters while maintaining a high speed, it could be the result of a worn out fuel pump.
For old engines, maintaining compression can be a bit of challenge. Compression-related issues are often a major contributing factor to a loss of power, and the fixes aren’t as easy as some of the other reasons, simply because metal components have worn over time. There are several different ways an engine can lose compression:
Worn Piston Rings
One of the major things that can happen over time is the piston rings will wear down, and this will allow for blow-by. Some of the high pressure air and fuel mixture combusting will pass by the pistons and travel along the cylinder walls into the crank case. This is pressure that should be pressing the piston down, so power is lost. It also means less compression as some of the air can escape as the piston travels upward on the intake stroke. With blow-by after combustion, the oil will contaminate much more quickly as the byproducts of combustion enter the crank case.
Carbon Deposits On Intake Valves/Valve Seats
If carbon deposits build up on the valves or valve seats, this can prevent the valves from closing properly. If an intake valve cannot fully close, it will allow air to escape during the compression stroke, effectively lowering the compression ratio. This could also result in backfiring as the air/fuel mixture travels past the intake valve during combustion. Exhaust valves that cannot close properly will also result in a lower effective compression ratio.
Carbon Deposits On The Piston
If deposits build on the piston or cylinder walls, these deposits can create hot spots. These hot spots can result in engine knock if conditions allow for it. If the engine is capable, it will retard the ignition timing to reduce the likelihood of knock. By retarding the ignition timing, power is lost.
Fouled Spark Plugs
Spark plugs can build deposits with time. Inconsistent spark plug firing means you’re likely to misfire. Maintaining clean plugs ensures that the spark part of the equation doesn’t result in a loss of power.
Change Your Oil Regularly
With good preventative maintenance, friction shouldn’t be a huge concern, but if you allow the oil to gunk up, small increases in the viscosity of the oil mean that it’s more challenging to pump (thus requires power to do so, which won’t be going to your wheels) or an engine that won’t have proper oil flow throughout the system. Without proper oil flow, components will wear much faster and will have to work harder to overcome the added friction. More wear, more friction, less power.
The best thing you can do to prevent power loss over time is to properly maintain your vehicle. Regardless, some power loss is unavoidable. Nothing lasts forever. Even kittens become dull with time.